In the known treatment of waste waters such as sewage to remove impurities therefrom by oxidative digestion, the sewage is aerated or oxygenated to promote the activity of micro-organisms therein which respire oxygen. It is desirable to maintain a relatively high dissolved oxygen (D.O.) concentration in the sewage and, in order better to achieve this, it is known to use an oxygen-enriched gas (for example commercial oxygen) instead of air. In practice, it is necessary to supply considerably more oxygen than can be dissolved in the sewage because the presence of dissolved N.sub.2 prevents the maximum concentration of dissolved oxygen being achieved and this is wasteful of the oxygen.
It is also known to use an oxygen-enriched gas to prevent the formation of hydrogen sulphide in sewers, particularly rising main sewers (see Progress in Water Technology, Vol. 7 (1975) No. 2, pages 289-300). Again, in order to achieve the highest desirable D.O. concentration levels in the sewage, more oxygen has to be supplied than is actually taken up and used by the micro-organisms in the sewage, and this is wasteful.
As will be understood by those skilled in the art, oxygen-enriched gas is a fairly expensive commodity and its wastage in conventional sewage treatment processes as above described is disadvantageous.
Furthermore, in other fields gas wastages tend to arise in trying to maintain a high concentration of dissolved gas in an aqueous liquid.